This invention relates to photovoltaic detectors and solar cells. Twenty years after the invention of the silicon solar cell, the photovoltaic solar power plant is still an elusive goal. About two orders of magnitude more capital is required for a silicon power station than for one burning fossil fuel ($0.40 per average watt). At fault are the processing costs resulting from the requirement for high quality bulk material and for shallow p-n homojunctions. Bulk silicon is needed because of the large absorption length (.about. 100 .mu.m) in this indirect band-gap material.
Alternatives to silicon involve direct-gap materials with large absorption coefficients. In our copending patent application, Ser. No. 522,060, filed Nov. 8, 1974 and assigned to the assignee hereof, we have disclosed our invention of a solar cell employing a heterojunction of direct-gap indium phosphide (InP) and cadmium sulfide (CdS).
Using the techniques described in that application, we have obtained actual solar efficiencies up to 12.5 percent; but we desire still better results.